i) Specific speed: The specific speed at which optimum efficiency occurs varies with the pump type. For example, the specific speed ns (Ns) for optimum efficiency for a volute-type pump is in the vicinity of 50 (2,500). With a vertical turbine diffuser type pump, the specific speed ns (Ns) for optimum efficiency is about 100 (5,000). Volute pumps selected for services with ns (Ns) values that are not in the vicinity of 50 (2,500) will probably have lower efficiencies. The relationship between the arithmetic efficiency correction and ns (Ns) is shown on Figure 20.3d below. The curve labeled "V" is for vertical turbine pumps, and the curve labeled "All but V" is for all other rotodynamic pump types.
To ensure high efficiency, the design specific speed must be close to optimum indicated by the curves.
For more detail on this subject, see the soon to be published guideline, HI 20.3 Rotodynamic (Centrifugal and Vertical) Pumps. Efficiency Prediction Method.
Q. What are "boiler feed booster pumps," and why are they necessary?
A. The following answer is taken from a soon to be published Hydraulic Institute Application Guideline for Power Plant Pumps.
Boiler feed booster pumps are used to provide pressure to the feed pumps to meet their NPSH requirements and avoid cavitation. As the size and speed of boiler feed pumps have increased, the NPSH requirements have increased as well. It is not practical to install the direct-contact heaters from which feed pumps take their suction at sufficient elevation to provide adequate NPSHa without "boosting" the suction pressure to the feed pumps. Using low-speed booster pumps ahead of the feed pumps increases suction pressure.
Boiler feed booster pumps are generally of the single-stage, double suction design. See Figure A.3, pump type BB1 for axially split case, and Figure A.4, pump type BB2 for radially split case versions of this design configuration. They operate at lower speeds than the feed pumps, typically at four-pole motor speeds. The NPSH required by the booster pumps is much lower than that required by the feed pump it supplies. It is not unusual for the NPSH requirements of large, high-speed boiler feed pumps to be in excess of 60 m (200 ft). Such a requirement is much more than could be economically provided by elevation differences from feedwater heater placement to the feedwater pump.
Figure A.3. Impeller between bearings—flexibly coupled, single stage, axial (horizontal) split case